Further purification and characterization of the steapsin-solubilized iodothyrosine deiodinase from bovine thyroid microsomes is planned, with special attention to the lack of responsiveness to NADPH, and to comparison of the properties (co-factor, molecular weight, kinetics, antigenic determinants) of the enzyme derived from thyroid with that which we hope to purify from liver and kidney. The effects of colchicine, which in vitro has been shown to disaggregate thyroid microtubules and impair endocytosis, iodine release, and hormone formation, will be studied in vivo, with particular attention to thyroid hormone formation and secretion in the rat and mouse. We hope to study dose-response relationship, the latency of any effects observed, the possible dependence of such effects upon protein synthesis, and the effect of colchicine and vinblastine upon the concentration of TSH and circulating thyroid hormones in the rat. The mechanism of chromate inhibition of glutathione reductase (yeast, thyroid) will be studied by documenting possible interaction of chromate and GSH that might produce a possibly inhibitory complex. In these studies, spectrophotometric assay of enzyme activity will be used, and reactions will be monitored by use of H3-GSH and Cr51-chromate, paper chromatography and gel filtration for separation and identification of reactants and products.